In the recent years, a variety of widely spread wireless communication networks has been developed. These networks use different access technologies and range from wireless personal area networks to satellite based networks, that are globally available. Between the above mentioned extremes, there is a variety of different networks and network technologies available that provide local, regional or country wide coverage, such as Bluetooth, Wireless-LAN using the IEEE 802.11 standard or cellular telecommunication networks such as GSM or UMTS.
Upon movement of a user, different network access technologies may be available to the user terminal. Small areas with a typical size of a building are often covered by networks using Bluetooth, DECT or Wireless-LAN (W-LAN) technologies. In a private environment the user e.g. favors using a headset or the like supporting Bluetooth technology or using the service provided by a W-LAN hotspot, that is typically available at airports, railway stations, internet cafes, etc.
In an urban environment the user terminal can be connected to a cellular network, whilst upon movement to a more rural area the service may be provided by a different network provider. So the network technology e.g. GSM or UMTS may remain unchanged but the access network and the service provider changes. Upon further location change, e.g. when the user crosses the borders of a country, the need for roaming between networks of different providers and even handovers between different network technologies very likely arises.
The user may move to areas where no network service is available except for satellite based network service. Also in this situation there may be the necessity to handoff the mobile terminal e.g. from a UMTS network to a satellite based network.
In addition, not only the handoff of mobile terminals between different access networks and different access technologies, also parallel communication with different networks, network technologies, or different access points may be of interest. Upon movement of a user to an airport or a railway station, it may be of interest to receive services e.g. an updated timetable via a local W-LAN hotspot service, while remaining connected to e.g. a voice service provided by UMTS.
In summary, nowadays a user terminal upon movement may utilize a variety of different wireless communication networks, network technologies and services that are available. It may be desirable to handoff the mobile terminal, to establish one or more connections and to keep the mobile terminal connected via one or more radio access points or radio access technologies.
Different networks and network technologies can be coupled with the aid of the Multi-radio Unification Protocol (“A Multi-Radio Unification Protocol for IEEE 802.11 Wireless Networks”, Adya et. al, Proceedings of the first international conference on broadband networks, BROADNETS'04). This protocol is applicable to heterogeneous wireless communication networks using e.g. IEEE 802.11 and UMTS access technology. In such a network it is not only desirable to handoff the mobile terminal easily across the various networks; the mobile terminal may also be capable of communicating with more than one radio access point at the same time, using the respective radio access technique the access point supports.
The option of parallel data transmission leads inter alia to an increase of reliability of the connection and a higher data throughput. A further advantage of parallel transmission is the optimized use of different bandwidths offered by the respective access point, i.e. the respective access technology. Data related to different applications such as voice service or data download may be provided to the mobile terminal using a radio access technology offering the appropriate bandwidth, e.g. IEEE 802.11 may be used for high speed data packet services and UMTS may be used for voice services.
It may be even possible that within one application, e.g. the audio stream is provided by the radio access technique offering the lower bandwidth and the video stream is provided by the radio access technique offering the higher bandwidth.
Additional to bandwidth also the data delay is desired to be within a certain range, appropriate to the specific application. Some applications e.g. data download are delay insensitive, other services, like Voice-over-IP (VoIP) or video streaming are not.
In radio access networks, transmission of the same data flow via different radio access points is possible at different transmission time intervals (TTIs). This possibility implies a very dynamic switching of the radio access point and in some cases switching of the radio access technology as well.
In order to provide best quality of service to the mobile terminal in these radio access networks, a multi-radio Automatic Repeat Request (ARQ) mechanism is typically implemented. The term multi-radio ARQ means that link layer acknowledgements for data packets can be transmitted via a different radio access point than the one used for the data transmission. The motivation for introducing multi-radio ARQ mechanisms, is to avoid cases where the radio access point switching periods may be very long. This might happen in some cases when data and acknowledgement transmission are done via the same radio access point, as the delay on uplink and downlink may be significantly different.
To optimize system throughput and to minimize the signaling overhead it is desirable to resend a data packet only when this is needed and only on that specific part of the link where it is absolutely required. Further, in order to prevent abnormal situations in the sense that e.g. the transmission of the data flow stagnates, it is of interest to send acknowledgement messages without considerable delays. This delay times should be at least not higher than the ones observed in case of absence of a multi-radio ARQ mechanism. A further appropriate measure to minimize the control signaling overhead may be to prevent duplicated transmission of acknowledgement messages or duplicated retransmission of data packets.
In FIG. 1, an example of an heterogeneous radio access network is presented. A mobile terminal is in communication with a first Radio Access Point RAP 1 and a second Radio Access Point RAP 2. The first Radio Access Point RAP 1 is represented as an UMTS/HSDPA Node B with more functions so as to support the MUP protocol. The second Radio Access Point RAP 2 is a Wireless LAN Access Point that supports the IEEE 802.11g technology. For the sake of the simplicity of explanation, the description below will refer to this network configuration and hence to these radio access technologies. However, other radio access technologies may be considered without departing from the idea of the invention.
The first Radio Access Point RAP 1 and second Radio Access Point RAP 2 are connected via a wired link, preferentially a cable enabling a high-speed data transmission. Hence, the delay introduced when transmitting data and signaling between the first Radio Access Point RAP 1 and second Radio Access Point RAP 2 can be very small. A part of the wired link is shared with other connections between the first Radio Access Point RAP 1 and other Wireless LAN Access Points such as the access points RAP3 and RAP4 represented in FIG. 1. Since the wired connection is partly shared between different connections and between uplink and downlink, the load in the shared part may vary. This results in a varying delay on the communication part between the first Radio Access Point RAP 1 and the second Radio Access Point RAP 2. For the same reason, the same applies for the switching and queuing delay that is introduced in the connector shown in FIG. 1.
In the example shown in FIG. 1, downlink transmission, i.e. transmission from the network to the mobile terminal is considered. It is possible to transmit data packets of the same user flow via different Radio Access Points across different transmission time intervals (TTI). Hence, it is possible to switch dynamically the Radio Access Point from which the data transmission is done. In the case of tight coupling, this Radio Access Point switching may be done at TTI level. In this radio network configuration, multi-radio ARQ mechanisms are possible, i.e. acknowledgments associated with the data flow transmission of a single user may be transmitted using a different radio access technology than the one used for the data transmission. For this reason, there is a single link layer buffer located at the MUP protocol of one of the various radio access points, RAP1 in FIG. 1. This buffer is responsible for data retransmissions and therefore the acknowledgments, in particular the negative ones, of the user should reach it. In the network configuration considered in FIG. 1, the MUP buffer is located at the first Radio Access point RAP 1 that is a Node B. Therefore, the acknowledgments at MUP level should always reach the first Radio Access point RAP 1, even if the data transmission is performed by the second Radio Access point RAP 2.
It is readily understood that in this configuration, the second Radio Access Point RAP 2 supports a minimal MUP functionality, so as to be able to communicate with the first Radio Access point RAP 1. Therefore, the second Radio Access Point RAP 2 is considered to be an evolved Wireless LAN Access Point. The first Radio Access Point RAP 1 and second Radio Access Point RAP 2 need to communicate with established rules and therefore a communication protocol is needed. Various inter-RAP communication protocols may be conceived. An example of an RAP Communication Protocol (RAPCP) is given in the co-pending European patent application 06 006 872.3. Therein, the main messages and procedures of this communication between a first Radio Access Point RAP 1 and a second Radio Access Point RAP 2 are described. Among others, therein the message with which packets to be transmitted are forwarded from the first Radio Access point RAP 1 to the second Radio Access Point RAP2 is defined. In addition, in this application, a procedure where only positive or negative acknowledgements are forwarded from the second Radio Access Point RAP 2 to the first Radio Access Point RAP 1 is described. Moreover, it is also defined therein that first Radio Access Point RAP 1 periodically polls the second Radio Access Point RAP 2 asking for information on the acknowledgments received by the second Radio Access Point RAP 2.
In addition, it is readily deduced that the station terminal supports both radio access technologies; so that the mobile terminal is able to implement both UMTS and IEEE 802.11 radio access technologies. It is assumed that at a given time interval, the station terminal is able to operate only in one radio access technology, so that simultaneous transmissions via both radio access technologies is not possible.
In the co-pending European patent applications 05 027 218.6 and 06 006 872.3 also due to the applicant, it is proposed that a Radio Access Point to which a mobile terminal sends an acknowledgment message be selected within the mobile terminal receiving data packets. The mobile terminal determines the Radio Access Point to which it sends an acknowledgment message based on the delay that the acknowledgment experiences when it is transmitted to the Radio Access Point with the MUP buffer and on the channel quality of the channels to the various Radio Access Points. In order to make this selection, the mobile terminal needs to have information on the delay and on the channel quality on the various Radio Access Points. This information is transmitted over the air to the mobile terminal. However, in the respective proposed systems, the signaling overhead over the air is relatively high and the information on the transmission delays on the various paths that is available may often be inaccurate, thus leading to a non-optimal selection of the Radio Access Point to which an acknowledgment message should be transmitted.
In a scenario where multi-radio ARQ mechanisms are implemented, the need for a timely transmission of acknowledgments is more intense. The reason is that access systems of more than one radio accesses are expected to lead to higher data rates than in single radio systems and therefore the associated acknowledgments need to be transmitted with a lower delay than the one needed in single-radio systems. Otherwise, delayed acknowledgments may stall the data transmission. The co-pending European patent application 05 027 218.6 presents a mechanism wherein a multi-radio terminal station selects the Radio Access Point to which the multi-radio terminal station transmits its acknowledgment message. The radio access system scenario is the one presented in FIG. 1. The Radio Access Point selection is based on the channel quality that the different radio accesses exhibit and on the delay that the transmission of the acknowledgment experiences when it is transmitted via the different available paths.
The information that is needed at the station terminal is signaled over the air. A first indispensable parameter for the operation of this algorithm is the delay of the acknowledgment transmission in the wired part; i.e. on the wired link connecting the first Radio Access point RAP 1 with the second Radio Access Point RAP 2. This parameter is explicitly signaled to the multi-radio station terminal. There are two options for the signaling of the delay in the wired part.
The first option is that the delay of an acknowledgment transmission is measured very frequently in the wired part. In this case, this updated delay information is signaled to the station terminal, upon reception of a new delay value. This case involves significant signaling overhead. In addition, upon arrival of a new delay value, this last value is forwarded to the MUP protocol of the station terminal and the Radio Access Point selection algorithm takes place. The assessment of the delay that is involved in this forwarding of the new transmission delay via primitives inside the station terminal and the execution of the Radio Access Point selection algorithm is not straightforward. However, some additional delay has to be expected. This might be an additional effect that has to be considered in the case of frequent updates of the delay value.
The second option for the measurement of the delay in the wired part and for its signaling to the station terminal is to perform this procedure less frequently. In this case, the signaling overhead is not so significant. In addition, the delays involved by the forwarding of the delay transmission value and the Radio Access Point execution algorithm are avoided, since these procedures are performed only upon arrival of a new delay value. This second option is suitable in scenarios where the delay in the wire connecting the Radio Access Points is not varying very dynamically. However, in the case where the connecting wire is shared among different Radio Access Points and carries traffic and signaling on both directions, the delay is expected to vary dynamically. In case this second option is applied when the delay in the wire is varying dynamically, then unwanted situations like the one described in the co-pending European patent application 06 006 872.3 occur.
The European patent application 06 006 872.3 discloses a solution in the case where the value of the delay on the wired part that the station terminal uses for its Radio Access Point selection algorithm is not the one that is observed at the wired part at this current instant. Namely, this application deals with the situation where the current delay in the wired part is smaller than the delay value that the station terminal uses for the Radio Access Point selection algorithm. A second Radio Access Point RAP 2 is an Wireless LAN Access Point that operates according to the IEEE 802.11 technology. In this case, the second Radio Access Point RAP 2 forwards a negative acknowledgment to the first Radio Access Point RAP1 and the negative acknowledgment reaches the MUP buffer located in the first Radio Access Point RAP 1 before the acknowledgment that the station terminal transmits to the first Radio Access Point RAP 1, thus potentially leading to contradictory acknowledgments.